1. Field of the Invention
This invention relates to geophysical surveying by short-pulse radar systems, and particularly to improved signal processing techniques for use in such systems.
2. Description of the Prior Art
In the conventional sense of the term, radar is an active electronic system in which a measurement of the round-trip transit time of an electromagnetic radiation pulse (or a sequence of them) is used to determine the distance to the target from which the electromagnetic pulse was reflected. Through use of information present in the return waveform other than the round-trip propagation time delay, other target information may be obtained, such as its radial velocity or shape and size. The transmitted pulses usually are repetitive, but there may be (in effect) only a single pulse. If repetitive, the pulses are usually spaced periodically in time but need not be, in general.
The principal distinctions between a conventional radar and a short-pulse radar (SPR) are to be found in a comparison of the temporal and spectral characteristics of a single pulse from the transmitted sequence.
The conventional radar pulse typically is a tone burst. A tone burst is a segment, n cycles in duration, of a sinusoidal oscillation, which is, of course, characterized by a single frequency f.sub.o. The number n of cycles contained in the tone burst may range from 10 to 10.sup.5 cycles, or long enough for the pulse to have a clearly defined central frequency f.sub.o. The truncation of the sinusoid in time broadens the spectrum from that of a single component f.sub.o (i.e., a delta function spectrum) to one having a continuous band of frequencies .DELTA.f centered about f.sub.o. The time duration of the pulse (or alternatively, the large number of cycles n) are such that .DELTA.f is much smaller than f.sub.o. That is, the spectrum of a single pulse of a conventional radar occupies a relatively narrow bandwidth .DELTA.f which may be characterized by a central frequency f.sub.o.
The radiated pulse of a short-pulse radar is a wavelet characterized by only a few excursions or zero crossings (generally less than or approximately equal to 10), and by a time duration which, generally, is shorter than that of a conventional radar pulse. These temporal characteristics manifest themselves in an extremely broadband frequency spectrum which is not characterizable by a single dominant frequency or narrow range of frequencies. The wavelet can be sufficiently short in time to approximate a delta function, which is a mathematical construct having (loosely speaking) zero time duration, an amplitude approaching infinity, and a finite area. The continuous spectrum of a temporal delta function is the most broadband spectrum of all, in that is has a constant amplitude for all frequencies. For this reason, a short-pulse radar is sometimes called a delta-function radar.
Through employment of a combination of antenna and transmission line techniques, the temporal shape of the SPR pulse may be that of a bipolar wavelet having several zero crossings, a bipolar wavelet having only a single zero crossing, or a unipolar field followed (after a controllable time delay) by a single unipolar field pulse of opposite sign (so as to have zero net area, and therefore no DC component in the spectrum of the total radiated pulse.
A short-pulse geophysical radar (SPGR) is a short-pulse radar applied to a geophysical problem. It is an electromagnetic short-pulse radar technique for remotely sensing, from the surface of the earth or above or from within mines and tunnels, the presence and location of subsurface geological features and buried artifacts.
Examples of such subsurface geological features include the interfaces between geological strata, the material of the strata, the presence of boulders, rock, or aggregate, the depth through overburden to bedrock, the presence and extent of cavities or voids in limestone or other materials, and the depth to the water table. Examples of such buried artifacts include metallic or non-metallic utility pipes, conduits, and lines such as might be used for water, gas, and sewage distribution, and other buried metallic and non-metallic objects.
The SPGR technique is applicable also to remotely sensing the presence and location of geological features and artifacts covered in whole or in part by fresh water or ice. Application of the technique to remotely sensing the presence and location of geological features and artifacts covered in whole or in part by salt or certain types of brackish water requires a modification of the prior art systems.
An SPGR system may be conveyed by any appropriate surface means (such as a cart, wagon, sled, hovercraft, automotive vehicle, or water craft) or by an airborne platform (such as a helicopter or other suitable aircraft). The method of conveyance is primarily a matter of convenience since the remote-sensing method described herein is only secondarily dependent in its operation upon the type of conveyance. The only dependence is upon the speed of the conveyance.
FIG. 1 is a general block diagram of prior art SPGR systems. A transmitting antenna 2 is used to radiate short wavelets of electromagnetic energy. A triggered pulser 4 excites the transmitting antenna, and is driven by a driver 5. A receiving antenna 6 senses the electromagnetic energy returned from subsurface interfaces and targets and converts the energy into a voltage, current, or charge return waveform. A wideband amplifier follows the receiving antenna and amplifies the return signal waveform. A time-variable gain element (analog gate 10 or amplifier) follows the receiving antenna, preceding (and, perhaps included with) with wideband amplifier, and is used to protect the receiver chain from high-level, transmitter-induced, non-information-bearing transients. A sampler 12 or sampling oscilloscope converts the information-bearing, short-duration return waveform into an analog replica which is lengthened or stretched in time such that it can be utilized directly by a slow, small-bandwidth record/display device 14, such as a paper chart, helix recorder or a raster-scan oscilloscope. A system controller 16 triggers various system elements and synchronizes the flow of pulses and signal return information among the system elements. The time-variable gain element 10 (labelled analog gate) may have a gain vs. time characteristic such that the gain increases with time to emphasize the later portions of the return signal waveform. Whereas the configuration of FIG. 1 employs separate transmitting and receiving antennas, FIG. 2 illustrates a similar system in which a single antenna 18 is time-shared for both transmitting and receiving through the use of a hybrid network 20.
Alternate prior-art SPGR-like systems utilize some form of sampling to slow down the return signal waveform resulting from a delta function (i.e., a very short duration) stimulus. The system modifications and improvements of this invention are applicable to such systems as well.
The prior art SPGR systems, such as shown in FIGS. 1 and 2, suffer in common from certain serious limitations.
There is a less-than-adequate signal-to-noise ratio in the SPGR return waveform (which carries the information relative to the subsurface targets) due to physically unavoidable severe attenuation and dispersion of the SPGR pulses within the earth. The undesirble effects of this attenuation and dispersion are further compounded by the wide bandwidth required in the receiving electronics to preserve and utilize the information content of the SPGR return waveform in the time domain (as opposed to a frequency domain analysis or display of the information in the return waveform). Since the signal bandwidth must be large (several hundred MHz, almost as large as presently available technology will permit), the noise bandwidth is also large. It is a fundamental electronics axiom that, for a given amplifying system, one obtains increased bandwidth at the expense of decreased gain and increased noise level. A less-than-adequate signal-to-noise ratio in the information-bearing return waveform places serious limitations upon the depth accessible by the technique, upon the usable or effective resolution at a given depth, and upon the operator's capability to unambiguously identify subsurface features.
There is radio-frequency interference (RFI) which occurs due to operation of the SPGR transmitter at high peak power levels, because of an effort to improve the signal-to-noise ratio of the information-bearing return waveform. Although operation of the SPGR at high peak power levels (kw or greater) would not constitute an inherent limitation, the broad spectral bandwidth of the SPGR pulses, and the presence of other users of the available radiofrequency spectrum, such as the communications services and civil aviation, cause legal limitations of available powers to control RFI. Thus such high-power operation could be in violation of FCC regulations, and also might represent a hazard to human life and safety.
There is less-than-adequate resolution along the traverse lines of the SPGR due to geometric spreading out of the radiated wavefronts as they propagate from the transmitter antenna. This leads to the phenomenon of hyperbolics, which are also often seen in acoustic subsurface profiling records wherein a small subsurface target appears to have a hyperbolic shape with the displayed hyperbola opening downwards. This results in a decrease in the effective horizontal resolution since two or more such targets closely spaced may produce a record which is not readily distinguishable from that of a single target.
There is an interpretational ambiguity of the information in the displayed return waveform (as made visible by the system record/display device) due to the fact that a SPGR reflection from a single subsurface interface (geological or otherwise) will manifest itself in the return waveform (and thus in the display) as closely-spaced, multiple interfaces. For example, in the case of the commonly-used helix recorder whereon a record of interface location versus depth is presented, a single interface will be written not as a single point or line, but as closely-spaced, multiple points or lines. Such ambiguities, while recognizable as such under certain favorable conditions, can effectively reduce the resolution with depth since it is not known whether one or a multiplicity of target interfaces has led to the displayed multiple indication. The origin of this undesirable effect lies in the relation between the waveform applied to the transmitter antenna and the resulting electromagnetic field, the propagation characteristics of the fields through or from the interfaces, and finally, the relation of the returned electromagnetic field to the waveform produced at the receiver antenna terminals. It is the signal-return voltage waveform which is displayed in linear or in rectified and compressed fashion.
There is a complete lack of ability of the prior art SPGR systems to operate in salt water so as to obtain information concerning the presence and depth of sub-bottom geological interfaces and artifacts. This lack of ability to operate through salt water is due to the high conductivity of salt water leading to attenuation so severe as to render the technique useless.